The device font in question is _sans, not Arial (which isn't a 'legal' name for a device font, though the pp at least on Linux is quite happy with it).

I installed the fonts from http://www.gnu.org/software/freefont/ (hosted on Savannah) and modified the SWF to look for FreeSans instead of _sans. Lo and behold, both Gnash and the pp then display Japanese characters!

So it looks like the answer to the problem here is (a) to install a font with enough characters and (b) to make sure fontconfig finds that one and not another.

(a) is tricky as freefont, which seems to be the most advanced free unicode font project, still has missing characters even in Japanese (and many character sets aren't implemented at all). I don't know that much about fonts, but FreeSans doesn't look all that attractive to me either. It's not a problem unique to Gnash, but it might be worth providing advice on installing fonts if a suitable source can be found.

(b) is the part relevant to Gnash. Not sure if fontconfig makes it possible to influence font matching criteria. I couldn't force it to load FreeSans at all.

The two non-English texts start with a 3-byte unicode identifier that confuses Gnash (known bug). When this is removed, the German text displays but the Japanese one doesn't. On the mailing list it was reported that the Adobe player on the same system displays the Japanese text. The device font is specified as Arial.

On my system, Gnash loads the font LiberationSans-Regular.ttf, which contains glyphs for only a couple of thousand characters: no Japanese. Other applications (Abiword, OpenOffice.org) that use these fonts are also incapable of displaying the Japanese. The system font and web browsers seem able to do it.

However, the Adobe Player also fails on my system, which suggests there is no available scalable device font that contains Japanese glyphs. In that case, Gnash's behaviour is not a bug.

The fact that the PP and Gnash behave differently on another system suggests that a scalable unicode font is installed (e.g. the very-hard-to-find and proprietary Arial-Unicode-MS), which isn't selected by Gnash, but is by the PP. This might be a bug (selection of an inadequate font when an adequate one is available), but until I get hold of such a font I can't tell.

There appears to be no font containing anything like a majority of unicode characters under a free licence.